1. Field of the Invention
The present invention relates to upgrading iron ore to decrease the amount of nonferrous materials therein, and to thereby increase the iron content thereof. More particularly, the invention relates to a process utilizing magnetic fields to separate a significant amount of non-magnetic material, such as silica or pyrolusite, from valuable iron oxide in an iron ore material. The invention therefore provides an improved source of iron oxides for high purity uses such as, for example, in direct reduction processes or heavy media coal beneficiation.
2. Discussion of Related Art
Treatment of ores in general to concentrate their valuable constituents (minerals) into products (concentrate) of smaller bulk, and simultaneously to collect gangue into discardable waste (tailings) is referred to as "ore dressing." The fundamental operations of ore dressing processes are the breaking apart of the associated constituents of the ore by mechanical means (severance) and the separation of the severed components (beneficiation) into concentrate and tailing, using mechanical or physical methods which do not effect substantial chemical changes in the ores. Beneficiation therefore consists of two fundamental operations: the determination that an individual particle is either a mineral or a gangue particle (selection); and the movement of selected particles via different paths (separation) into the concentrate and tailing products. Selection is based upon some physical or chemical property in which the mineral and gangue particles differ in kind or degree or both.
Known beneficiation techniques include, for example, flotation, gravity settling, electrostatic separation, or other special processes. Further, the use of magnetic separators to remove ferromagnetic minerals is generally known. Examples of processes and devices for magnetic separation include, among many others, those disclosed in U.S. Pat. No. 3,045,822 to Cavanagh, U.S. Pat. No. 5,636,748 to Arvidson, U.S. Pat. No. 4,512,879 to Attia et al., U.S. Pat. No. 4,307,225 to Bingel et al., U.S. Pat. No. 4,166,789 to Imai et al., U.S. Pat. No. 4,051,023 to Fogle and U.S. Pat. No. 3,502,271 to Hays. These patents are incorporated herein by reference in their entirety. However, to date methods of beneficiating iron ore by removing therefrom non-magnetic material (such as, for example, silica and pyrolusite) have not produced ore concentrates having satisfactory purity.
Turning now to a particular use of high purity iron ore concentrate, iron ores of varying purity are reduced to elemental iron using a wide variety of methods as a step in steel-making processes. Historically, reduction has been achieved in a blast furnace by heating the reactants to extremely high temperatures, thereby producing molten products of elemental iron and contaminants. The contaminants, or "slag," are then separated from the molten iron to yield a purified "pig iron" product. However, blast furnace processes are problematic because a they require huge capital investment, a new installation estimated to cost in the hundreds of millions of dollars. Major reline/rebuild projects are also required approximately every 8-10 years, these projects commonly costing from about 50 million to about 100 million dollars. Furthermore, conventional blast furnace processes also require production of coke as a starting material, and significant investment is required in a coke-producing installation to satisfy environmental standards in the United States.
A reduction method currently receiving a great deal of attention, due in part to the significantly lower capital costs associated therewith, is direct reduction of iron. Direct reduction is a type of reduction process in which the reduction reaction is achieved without melting the reactants. Skilled artisans in the field of refining iron are increasingly recognizing direct reduction as a useful method of converting iron ore into elemental iron. The two general categories of direct reduction are (1) those that utilize a gas such as, for example, natural gas as the reducing agent, and (2) those that utilize solid carbonaceous materials such as coal as the reducing agent (solids-based direct iron reduction). While solids-based direct iron reduction is presently being given a great deal of attention as a potentially useful reduction mechanism, gas-based processes are much more prevalent commercially. Processes which utilize natural gas as the reductant typically involve expensive oxide pellets or lump ore as feed stock. It is believed that the only solids based direct reduced iron currently being produced in any significant amount involves the use of oxide pellets or lump ore together with sized coal as the feed material. Irrespective of whether a gas-based or solids-based process is used, however, there is a great need for iron ore starting materials having significantly reduced gangue content.
In direct reduction, the starting materials are heated to a temperature below the melting point of the starting materials, but high enough to elicit reduction of iron oxides therein to yield "sponge iron." The term "sponge iron" refers to the product of a direct reduction process and is used interchangeably herein with the terms "direct reduced iron" and "DRI". The sponge iron then may be densified by briquetting or melted to further reduce iron oxide and extract the reduced elemental iron from contaminants such as silica, alumina and sulfur, which arc tightly bound to the elemental iron in the sponge iron product.
One significant problem encountered in direct reduction processes, is that a large input of resources is required to remove the contaminants from the elemental iron after direct reduction. In this regard, many iron ore starting materials available on the market, such as specular hematite concentrate, have a silica content of up to about 6% by weight. To produce a DRI product having an acceptable silica content, the ore used as the starting material in a direct reduction process should be no greater than about 3%, more preferably no greater than about 2%. Therefore, much effort has been expended developing improved processes for upgrading iron ores efficiently and on a large scale to increase the iron content, and decrease the silica content thereof.
Pyrolusite (MnO.sub.2) is another contaminant present in a number of iron ores available in the marketplace, and it is highly desirable to remove pyrolusite from an ore when it is present. For example, Wabush Mines, a company which mines specular hematite in the northeastern region of Canada known as the "Labrador Trough," offers for sale specular hematite (in the form of fines or as pellets) which has a manganese content of from about 1% to about 2% by weight, the manganese being primarily in the form of pyrolusite. Pyrolusite is a harmful contaminant in a feedstock for reduction and/or steel-making processes because it is resistant to reduction and manganese oxides are highly corrosive to refractories present in, for example, melting or smelting furnaces. Therefore, there is also a need for a process which removes pyrolusite from an iron ore. As the demand for DRI increases in North America and worldwide, the need for iron ore concentrates having significantly lower silica and manganese content also increases. Therefore, there is a need in the art for improved methods of upgrading iron ore and iron ore concentrates to achieve satisfactory levels of purity.
The present invention addresses the above-mentioned problems by teaching a process for upgrading iron ore having therein a nonmagnetic contaminant such as silica and/or pyrolusite to produce an ore having excellent purity for use, for example, in direct reduction processes or in heavy media coal beneficiation. By utilizing inventive methods, a high purity iron ore concentrate is provided which may be reduced, for example, by direct reduction, to thereby produce a DRI product which has a reduced amount of contaminants therein, this advantageously reducing the capital and operating costs associated with steel-making processes.